Isolated cow eyes will be used to study changes in the diameter of retinal arteries, veins, and capillaries when the blood is oxygenated within physiological limits. Three blood parameters, pO2, pCO2 and pH, will be varied in order to determine what values may alter vessel diameter. Information gained in the study will contribute to a better understanding of the physiological role of the blood parameters in the eye and, ultimately, a better understanding of vascular ocular diseases such as diabetic retinopathy.